The present invention refers to a process for magnetizing the magnets mounted to the metallic core of a brushless electric motor rotor and, more particularly, to a process for assembling the motor of a hermetic compressor.
Motors with permanent magnets in the rotor have been largely used in industrial applications and in household equipments, particularly electric appliances, consisting in the preferred solution for hermetic compressors of variable capacity.
These electric motors have permanent magnets usually in the form of arcuated plates retained to a cylindrical rotor core (usually in laminated or massive iron) mounted around the motor shaft. In these rotors, the magnets may be disposed in housings defined by longitudinal openings inside the metallic core, or maintained seated on the cylindrical lateral surface of said core, through different retaining means designed to impart to the mechanical structure the required resistance to the rotational and operational forces of the motor.
Since the equipments are produced on a large scale, the manufacturing process of these motors requires specific cares to avoid contamination with metallic particles, as well as handling cares.
In the known prior art, the magnets are individually magnetized before they are mounted to the rotor, or they are mounted to said rotor and magnetized afterwards, before the rotor is mounted to the electric assembly or equipment to which it will be applied.
The magnetization of a magnet for its posterior utilization in a magnetic device, whichever the conception thereof, is effected by applying to the virgin magnet a magnetizing field which is usually larger than the coercitive field (Hc) of said magnet.
This magnetizing field should have flux lines in the desired direction of magnet magnetization, since the magnetic field acquired by the magnet after magnetized will have flux lines in the same direction of the field which generated said magnetization.
The magnetizing field is usually obtained by applying a current pulse of high intensity and short duration to a ferromagnetic structure containing a coil, or a group of coils, in order to generate a magnetic field which, on its turn, magnetizes the magnet.
In certain electromagnetic devices, as for example the motors with permanent magnets in the rotor and used in hermetic compressors of variable capacity, the magnetic fields generated by the electric currents are very similar, in shape, to the optimum desired magnetic field in the magnet. The flux lines of the magnetic field generated by the currents have the same direction and orientation as the optimum magnetic field of the magnets. The difference lies on the fact that the amplitude of the field generated by the currents under operation is much lower than that required to magnetize the magnets.
However, it is possible to increase the current instantaneously, until the field reaches high values in short time intervals, without damaging the stator.
One of the consequences resulting from magnetizing the magnets in the way it is presently effected is related to the potential of magnetic attraction of said magnets onto solid particles during handling, transportation or storage of the rotor, before the latter is assembled to the equipment to which it is designed.
This magnetic attraction may also act in relation to the adjacent parts, whether they are magnetized or not, of the product to which said magnets are designed, and to those parts provided during the magnetizing process of said magnets, with the risk of mechanical impact and damage to the product.
These occurrences may impair the operation of the equipment, reducing its useful life and even damaging its structure or the magnets, either at the instant of an accidental impact or at the beginning of the operation.
For the hermetic compressor motors, particularly those of variable capacity, the problems resulting from the presence of loose solid particles inside the compressor are particularly relevant and may impair the useful life of the compressor, as well as the operational capacity thereof.
The loose solid particles attracted by the magnets, already magnetized when the rotor was assembled to the compressor, during the operation of said compressor collide with the bearing and the piston-cylinder unit, producing new loose particles which provoke wear in the bearing.
The loose solid particles inside the compressor reach the oil sump and, besides being conducted to all parts lubricated by the lubricant oil in the compressor (motor, compressor inside) and colliding, for example, with the bearing and piston-cylinder unit, are also conducted, together with the refrigerant gas, to the refrigeration circuit to which the compressor is associated, resulting in the partial or total obstruction of the capillary tube of the refrigeration system, impairing the refrigeration efficiency of said system and, depending on the obstruction degree, impairing the refrigeration operation of the compressor. Another disadvantage of the prior art of magnet magnetization for electric motor rotors of hermetic compressors relates to the consequences resulting from the disalignment of the magnetic fields of both the rotor and stator, which commonly occurs in this technique.
When the assembly of the eccentric shaft-rotor unit (the rotor carrying the already magnetized permanent magnets) to the stator does not provide an alignment of the rotor end faces in relation to the stator end faces, a disalignment occurs between the respective magnetic fields, originating a system of forces acting on the bearing and conducting to an operational unbalance of the eccentric shaft in the cylinder block and, consequently, between the piston and cylinder where said shaft operates, causing wear in said parts and also operational inefficiency of the compressor.
Besides these problems, the technique employed nowadays requires a specific equipment for generating and applying the magnetizing field to the magnets mounted to the rotor, but before said rotor is mounted to the motor assembly of the compressor.
Thus, it is a general objective of the present invention to provide a process for magnetizing the permanent magnets of an electric motor rotor, which eliminates the possibility of occurring magnetic attraction of the rotor magnets onto particles or adjacent parts of said magnets during handling, transportation and storage of said rotor, before the latter is mounted to the motor or equipment to which it is designed, as well as during the assembly operations of the rotor to the equipment.
Another objective of the present invention is to provide a process for magnetizing the rotor magnets, which does not require specific equipments for obtaining the magnetization of the rotor magnets.
A specific objective of the present invention is to provide a process for magnetizing the rotor magnets, which utilizes the motor structure with the rotor already mounted inside the stator.
A further objective of the present invention is to provide a process for magnetizing the magnets of an electric motor rotor for a hermetic compressor which, besides the advantages above, minimizes the presence of loose particles inside the compressor and the consequences related to the wear of the component parts thereof and to the obstruction of the capillary tube of the refrigeration system to which said compressor is associated.
These and other objectives are attained by a process for magnetizing the permanent magnets of an electric motor rotor, having rotor magnetic sectors, each with a single and respective magnetic orientation and comprising at least one magnet, and whose stator carries a plurality of coils, said process comprising the steps of: a- assembling the electric motor, by operationally positioning the rotor in relation to the stator; and b- promoting the controlled energization, from an external current source, of at least one coil of the stator, in order to promote the magnetization of the magnetic sectors.